1. Field of the Invention
The present invention relates to an apparatus for recording information on a recordable optical record carrier by irradiation of a light beam onto said record carrier for forming marks and lands representing said information along an information recording direction. The present invention relates further to a corresponding recording method, an optical record carrier and a computer program for implementing said method.
2. Description of the Related Art
The total data capacity of optical discs is determined by the radial and tangential data density. The radial density is determined by the data track pitch, the tangential data capacity by the shortest mark that can be written. For rewritable phase-change discs, re-crystallization at the trailing edge of a mark during writing of the next mark is used to obtain a mark of shorter length than the optical spot size. Typically, half of the mark is erased to end up with a crescent shaped mark. For the recently introduced Blu-ray Disc (BD), a total data capacity of 25 Gbyte can be recorded on a single recording layer of a 12 cm disc. The shortest length is 150 nm (d=1 code) while the optical spot is 300 nm in width (1/e radius of the optical spot is 150 nm).
A recordable (write-once) BD format is required in addition to the RW (rewritable) format. This so-called BD-R system should also achieve the high data-capacity of a BD RW system, i.e., 25 GB, to allow one-to-one copies. In recordable media, mark formation occurs when a threshold temperature is exceeded. The mark-formation corresponds to an irreversible change in the information layer, and effects comparable to the re-crystallization in rewritable media in principle do not occur. Thus, the marks that have been recorded in recordable media essentially reflect the optical/thermal profile that was present during the recording process. To reduce the mark length, thus to increase the tangential density, less laser power may be used to write a smaller mark. As a consequence, the modulation of these smaller mark, and thus signal-to-noise ratio, will drop as well. This can be similarly discussed for write-once media based on phase-change material as recording material. The as-deposited amorphous material will re-crystallize to form circularly shaped crystalline marks. Mark length reduction due to post heat is not possible.
U.S. Pat. No. 5,673,246 discloses an optical pick-up device which makes a numerical aperture of an object lens in a radial direction of an optical disc smaller than that in a tangential direction to a track on the optical disc. Thus, a spot diameter in the tangential direction to the track becomes smaller than that in the radial direction during read-out. Thus, it is possible to prevent degradation MTF (Modulation Transfer Function) as well as to accurately read data recorded on the disc.